The offset lithographic process is used for printing various mediums, which may be in sheet or web form. For example, the process is used in the metal can industry for printing product labels on sheets of steel, which typically run on the order of 30-40 inches square. Offset lithographic presses can be arranged two, three or more in a line so that multiple colors can be printed on a sheet or web during a single pass through a press line. In paper operations, 4-6 presses are common, while for metal decorating 2-3 presses are more typical. In order to print complex labels, the metal sheets frequently must be passed through the press line several times.
By the very nature of the offset lithography printing process, when a new job is set up, the press must be adjusted and stabilized before it is ready to produce an accurate and acceptable image on the printed material. Specifically, adjustments are made to the press color and/or registration during start up, after which a certain amount of waste material must be run inasmuch as it takes time for the effect of the adjustments to carry through to the printed material. This period of time is generally referred to as the "run-in" period. Stabilization of the press during the run-in period is further complicated as sheets are passed through multiple presses in a press line.
An offset lithographic press generally includes a combination of cylinders, usually a single plate cylinder, a blanket cylinder, and an impression cylinder, disposed in a parallel contacting arrangement. A press plate made of stainless steel, aluminum, or the like containing the image of art to be printed is mounted on each plate cylinder. Typically, an ink fountain along with ink distribution rollers supply ink to the surface of the press plate on the outer peripheral surface of the plate cylinder. In offset lithography a similar arrangement provides water to the surface of the press plate. The blanket cylinder, on which a printing blanket is mounted, transfers the ink from the press plate to the medium being printed. The printed material is fed through the nip between the blanket cylinder and the impression cylinder, which provides support for the medium to ensure accurate transfer of the ink image from the blanket.
During the run-in period noted above, it is necessary to operate the press in order to apply the ink and water to the press plate and transfer the image from the plate to the blanket in order to stabilize the press and the image on the printing blanket. During this period, ink will build up on the blanket and must be removed. This traditionally has been accomplished by running a sufficient amount of waste material through the press so as to stabilize the transfer of the ink image by the press. This cycle is repeated each time an adjustment to color or registration is made on the press. While running waste paper on a paper press may be costly to a printer, the cost of running start-off metal sheets in metal printing operations can represent a truly significant operating cost. Accordingly, in metal decorating there is usually an effort to use start-off sheets as sparingly as possible. Although for this purpose it may be possible to recycle metal sheets that have been spoiled in other operations, new sheets must frequently be utilized at significant expense. Further, as these sheets must be used sparingly, the commercial run is often begun before the press has fully stabilized. This can result in variations in print quality for the first 100-200 production sheets.
Offset lithography press users, and the can industry in particular, have attempted to reduce the costs associated with the use of expensive start-off sheets through various methods. As far as the start-up quality problems are concerned, very little hope has been given in the past for a solution. On both fronts, a practical and economical solution is required.